A typical millimeter waveband sensor includes a micro-circuit mixer connected via a waveguide to an aerial collecting dish. This micro-circuit usually consists of a dielectric support plate having patterned conductors and bonded semiconductor mixer components--e.g. diodes--on one surface and may also be metallised over all or part of the other surface. The positioning of these components is extremely critical. The micro-circuit is usually mounted in the waveguide cavity or else is connected to the waveguide by a specially designed transition. The positioning of this circuit relative to the waveguide is also critical. The accurate positioning of the components and the positioning of the micro-circuit are demanding mechanical tasks and are largely responsible for the relatively high production cost of these conventional sensors. These sensors have been considered difficult to set up, fragile and expensive.